Geology

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Science  22 Apr 2011:
Vol. 332, Issue 6028, pp. 399
DOI: 10.1126/science.332.6028.399-b

Occupational asbestos exposure is usually associated with one of two classes of minerals: amphibole and serpentine. Of these, amphibole asbestos is generally regarding as by far the more hazardous. At the same time, amphibole minerals are present in a variety of rocks. Although the asbestiform habit is relatively rare and restricted to specific occurrences and types of amphiboles, many amphiboles have an elongated habit that fits a generic definition of fibers used in many regulations, namely any particle longer than 5 µm and with at least a 3:1 aspect ratio. Most exposure to amphibole asbestos has been in mining or other occupational settings where the fibers are disturbed, but natural exposure is a persistent fear, and attempts at remediation could be enormously expensive. To assess the natural occurrences, Thompson et al. analyzed a U.S. Department of Agriculture soil database sampling tens of thousands of soils from across the United States. Amphiboles were widespread, occurring in about 13% of all samples and in samples from every state except Rhode Island (though it is likely that some amphibole is present locally there too). Separate checks on habits imply that, by the occupational hazard definition, many soils in the United States should be considered contaminated with asbestos, including many farmlands, which are regularly disturbed. This census illustrates the difficulty in extrapolating the occupational definition to mineralogic contexts.

Am. Mineral. 96, 609 (2011).

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